Track Points Behavior in Motion - using with Mocha track data?

[note; this is a crosspost from the Motion forum, but I figure a lot of X folk are good with Motion as well, so perhaps might get an answer here, as the traffic's higher]

So I need to animate via tracking data only one point on a B-spline. Been playing with Track Points Behavior, and I get how it works when I allow the behavior to handle all the tracking work. But how do I bring in a finished track from Mocha, and apply that to just one point on the spline? As soon as I change the behaviour source to the layer output from Mocha (it's called Surface), the individual shape's point track controls disappear, and it simply match moves the entire shape to the source well.